diff --git a/AGENTS.md b/AGENTS.md
index 0c248edda..ac84585d1 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -303,4 +303,4 @@ Agents poll `eval:bg:*` logs for the `### gstack-detach EXIT=<code> ###` sentine
 - Every Codex run's verdict goes through `lib/outside-review-result.ts`; its reason codes live in `lib/gate-outcomes.ts`, each with a `docs/troubleshooting.md` anchor.
 - `test/module-size-ratchet.test.ts` keeps refactored owner modules at or under 800 lines (150 per function) and residual files from growing.
 - The `claude` CLI binary resolves via `lib/claude-bin.ts` (re-exported from `browse/src/claude-bin.ts` for browse internals; `Bun.which()` + `GSTACK_CLAUDE_BIN` override). Set `GSTACK_CLAUDE_BIN=wsl` plus `GSTACK_CLAUDE_BIN_ARGS='["claude"]'` to run Claude through WSL on Windows.
-- Every Bun gstack starts ignores the working directory's `.env` files and `bunfig.toml`: bun shebangs are `#!/usr/bin/env -S bun --no-env-file --config=/dev/null`, skill prose passes `bun --no-env-file --config=/dev/null`, shipped shell files define `GSTACK_BUN_CONFIG` with the line in `test/bun-entry-isolation.test.ts` (`NUL` when `OSTYPE` is `msys` or `cygwin`, where native Bun cannot open `/dev/null`, #3415) and pass `bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}"`, TypeScript spawns use `bunIsolationArgs()` from `lib/bun-isolation.ts`, and `bun build --compile` adds `--no-compile-autoload-dotenv --no-compile-autoload-bunfig`. `test/bun-entry-isolation.test.ts` enforces all four.
+- Every Bun gstack starts ignores the working directory's `.env` files and `bunfig.toml`: bun shebangs are `#!/usr/bin/env -S bun --no-env-file --config=/dev/null`, skill prose passes `bun --no-env-file --config=/dev/null`, shipped shell files define `GSTACK_BUN_CONFIG` with the line in `test/bun-entry-isolation.test.ts` (`NUL` when `OSTYPE` is `msys` or `cygwin` and `cygpath` is on `PATH`, where native Bun cannot open `/dev/null`, #3415) and pass `bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}"`, TypeScript spawns use `bunIsolationArgs()` from `lib/bun-isolation.ts`, and `bun build --compile` adds `--no-compile-autoload-dotenv --no-compile-autoload-bunfig`. `test/bun-entry-isolation.test.ts` enforces all four.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 40d3a733f..38e22797d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,21 +2,30 @@
 
 ## [Unreleased]
 
-**Windows: gstack's shell bins stop losing review, question, learning and timeline records when Git Bash path conversion is off (#3415).** Since GSTA-56 every shell bin started Bun with `--config=/dev/null`. Native `bun.exe` cannot open `/dev/null`: it exits 1 with `EINVAL` on Bun 1.3.10, 1.4.2 and 1.4.4 (measured on windows-latest). Git Bash hides this by rewriting the argument to `nul`, but only while MSYS path conversion is on. With it off (`MSYS_NO_PATHCONV` set to any value, even empty; `MSYS2_ARG_CONV_EXCL=*`; or Cygwin), `gstack-review-log`, `gstack-question-log` and `gstack-learnings-log` exited 1 with nothing written, and `gstack-timeline-log` exited 0 with nothing written.
+**Windows: gstack's record bins stop losing review, question, learning and timeline records when Git Bash path conversion is off (#3415).** Since GSTA-56 every shell bin started Bun with `--config=/dev/null`. Native `bun.exe` cannot open `/dev/null`: it exits 1 with `EINVAL` on Bun 1.3.10, 1.4.2 and 1.4.4 (measured on windows-latest). Git Bash hides this by rewriting the argument to `nul`, but only while MSYS path conversion is on. With it off (`MSYS_NO_PATHCONV` set to any value, even empty; `MSYS2_ARG_CONV_EXCL=*`; or Cygwin), `gstack-review-log`, `gstack-question-log` and `gstack-learnings-log` exited 1 with nothing written, and `gstack-timeline-log` exited 0 with nothing written.
 
 ### Fixed
-- **40 shipped shell files** (the bash bins, the Claude hook wrappers, `setup`, `scripts/build-cso.sh` and one migration) define `GSTACK_BUN_CONFIG` once (`NUL` when `OSTYPE` is `msys` or `cygwin`, as in Git Bash, MSYS2 and Cygwin, the check `gstack-doctor` already uses; `/dev/null` elsewhere) and start Bun with `--config="${GSTACK_BUN_CONFIG:-/dev/null}"`. Bun 1.4.2 and later, gstack's minimum, read `NUL` as an empty config whether or not conversion is on. The `:-/dev/null` default fails closed: a call that loses its definition still passes `/dev/null`, so POSIX is unchanged and Windows fails loudly as before, and never reads the repository's `bunfig.toml`. `bin/gstack-codex-probe` is sourced into the caller's shell, so it defines the variable inside the one function that starts Bun and still runs nothing at source time.
-- **`bin/gstack-timeline-log`**: invalid input still exits 0 with nothing logged (the hook-path contract). Its validator now exits 3 for invalid input, and any other non-zero exit, which is Bun itself failing, is reported on stderr and returned instead of being skipped as invalid.
-- Unchanged: bun shebangs and skill prose keep `--config=/dev/null`. A shebang cannot name a per-platform path, and with conversion off their POSIX script paths (`/c/Users/...`) fail in `bun.exe` anyway, so they work wherever they worked before. TypeScript spawns keep `bunIsolationArgs()`, and `scripts/ubicloud/` keeps `/dev/null` because it runs on Linux.
+- **40 shipped shell files** (the bash bins, the Claude hook wrappers, `setup`, `scripts/build-cso.sh` and one migration) define `GSTACK_BUN_CONFIG` once and start Bun with `--config="${GSTACK_BUN_CONFIG:-/dev/null}"`. The value is `NUL` only when `OSTYPE` is `msys` or `cygwin` (Git Bash, MSYS2 and Cygwin, the check `gstack-doctor` already uses) and `cygpath` is on `PATH`; everywhere else it is `/dev/null`. The `cygpath` check is a builtin `command -v`, so it forks nothing. It means an `OSTYPE=msys` inherited by a Linux or macOS bash keeps `/dev/null`, instead of passing a relative `NUL` that Bun there resolves to a file in the repository. Bun 1.4.2 and later, gstack's minimum, read `NUL` as an empty config whether or not conversion is on. The `:-/dev/null` default covers a call that loses its definition: it still passes `/dev/null`, so POSIX is unchanged and Windows fails loudly as before, and never reads the repository's `bunfig.toml`. `bin/gstack-codex-probe` is sourced into the caller's shell, so it defines the variable inside the one function that starts Bun and still runs nothing at source time.
+- **`bin/gstack-timeline-log`** and **`gstack-developer-profile --log-session`**: invalid input still exits 0 with nothing logged (the hook-path contract). Their validators now exit 3 for invalid input. Any other non-zero exit, which is Bun itself failing, is reported on stderr and returned instead of being skipped as invalid.
+- Unchanged: bun shebangs and skill prose keep `--config=/dev/null`, because a shebang cannot name a per-platform path. TypeScript spawns keep `bunIsolationArgs()`, and `scripts/ubicloud/` keeps `/dev/null` because it runs on Linux.
+- Not covered: callers that hand `bun.exe` a POSIX script path built from `pwd` or `$HOME` (the Claude hook wrappers, `autoplan/bin/phase-publication-hook`, `plan-ceo-review/bin/mode-handoff-hook`, and some `gstack-doctor`, `gstack-codex-probe`, `setup` and migration calls) only got the config spelling. With path conversion off they may still fail on the `/c/...` script path itself. The record bins convert their own paths with `cygpath -m`; the others were not verified on Windows with conversion off.
 
 ### Tests
-- `test/bun-config-shell-bins.test.ts` (new; force-included in the Windows lane): on every platform, the four record bins run through `lib/script-invocation.ts` with conversion off in a repository whose `bunfig.toml` preload and `.env` are hostile. Each writes exactly one record, and neither the preload nor the `.env` state root takes effect. Invalid timeline input still exits 0 silently. On POSIX hosts a simulated Windows bash (`OSTYPE=msys`, and a `bun` stand-in rejects `/dev/null` with EINVAL as `bun.exe` does) reproduces the bug: on main the four bins write nothing and the timeline control exits 0 (5 failures); with the fix they pass. The controls, without `OSTYPE=msys`, show each bin exits non-zero with nothing written.
+- `test/bun-config-shell-bins.test.ts` (new; force-included in the Windows lane):
+  - On every platform, the four record bins run through `lib/script-invocation.ts` with conversion off in a repository whose `bunfig.toml` preload and `.env` are hostile. Each writes exactly one record, and neither the preload nor the `.env` state root takes effect. Invalid timeline input still exits 0 silently.
+  - On POSIX hosts a simulated Windows bash (`OSTYPE=msys`, a `cygpath`, and a `bun` stand-in that rejects `/dev/null` with EINVAL as `bun.exe` does) reproduces the bug: on main the four bins write nothing and the timeline control exits 0 (5 failures); with the fix they pass. The controls, without `OSTYPE=msys`, show each bin exits non-zero with nothing written.
+  - An inherited `OSTYPE=msys` with no `cygpath`, in a repository that commits a file named `NUL` with a preload, leaves each bin on `/dev/null`: the record is written and the preload never runs. This fails on the first version of this fix (4 failures).
+  - `gstack-developer-profile --log-session` reports a rejected config on stderr and exits non-zero, and still skips invalid input silently. This fails on main.
 - `test/bun-isolation-launch.test.ts`: the shell-file spelling stays isolated with conversion off, and a win32-only control shows that the literal `--config=/dev/null` fails there with EINVAL.
 - `test/bun-entry-isolation.test.ts`: shipped shell files must use the fail-closed spelling and define `GSTACK_BUN_CONFIG` with the canonical line before first use. Skill sources keep the literal spelling.
-- `test/learnings-injection.test.ts`, `browse/test/learnings-injection.test.ts` and `test/setup-hook-canonical-paths.test.ts` now anchor on the new spelling. Their assertions are unchanged.
+- `test/learnings-injection.test.ts`, `browse/test/learnings-injection.test.ts` and `test/setup-hook-canonical-paths.test.ts` now anchor on the new spelling. The two learnings-injection files also check that the anchored `bun -e` block exists. Before, a missing anchor sliced the script's last character and the negative assertions passed without checking anything.
 
 ### Gate integrity
-- `test/bun-entry-isolation.test.ts` is strengthened. On POSIX the required shell spelling still resolves to `/dev/null`. The literal `--config=/dev/null` in a shipped shell file is now an offender, as is an undefined or redefined `GSTACK_BUN_CONFIG`. The shell-caller detector also skips a `--config` value before deciding whether a call starts a program, so `bun --no-env-file --config=<anything> -e` no longer passes unchecked; the tree has no such call. Negative controls: the old spelling, a variable without the default, an empty `--config=`, a missing, late or altered definition, and a reverted bin each fail the check. No threshold, budget, timeout, trial count or case kind changed.
+- `test/bun-entry-isolation.test.ts` is strengthened. On POSIX the required shell spelling still resolves to `/dev/null`.
+  - Now offenders: the literal `--config=/dev/null` in a shipped shell file, and an undefined or redefined `GSTACK_BUN_CONFIG`.
+  - The shell-caller detector also skips a `--config` value before deciding whether a call starts a program, so `bun --no-env-file --config=<anything> -e` no longer passes unchecked. The tree has no such call.
+  - Negative controls, each of which fails the check: the old spelling, a variable without the default, an empty `--config=`, a missing, late or altered definition, and a reverted bin.
+  - No threshold, budget, timeout, trial count or case kind changed.
 
 ## [1.91.223.0] - 2026-10-25
 
diff --git a/autoplan/bin/phase-publication-hook b/autoplan/bin/phase-publication-hook
index 0bde64eb7..c88b95e29 100755
--- a/autoplan/bin/phase-publication-hook
+++ b/autoplan/bin/phase-publication-hook
@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # Native Read barrier. A broken install must return deny JSON, never silence.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 _AUTOPLAN_DECIDED=''
 _autoplan_backstop() {
   if [ -z "$_AUTOPLAN_DECIDED" ]; then
diff --git a/bin/gstack-brain-sync b/bin/gstack-brain-sync
index de78eed05..0c5d07afb 100755
--- a/bin/gstack-brain-sync
+++ b/bin/gstack-brain-sync
@@ -77,7 +77,7 @@
 BASH_COMPAT=50
 
 set -uo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 . "$(dirname "$0")/gstack-state-root.sh" 2>/dev/null || { echo "$0: cannot resolve the gstack state root: $(dirname "$0")/gstack-state-root.sh is missing. fix: reinstall with ./setup or /gstack-upgrade (docs/state-root.md)" >&2; exit 1; }
 gstack_state_root_select
diff --git a/bin/gstack-browser-ensure b/bin/gstack-browser-ensure
index f50d04533..764995fbb 100755
--- a/bin/gstack-browser-ensure
+++ b/bin/gstack-browser-ensure
@@ -24,7 +24,7 @@
 # GSTACK_CHROMIUM_NO_SANDBOX=1 for the probe. Bash 3.2 plus POSIX tools.
 BASH_COMPAT=50
 set -u
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)" || { echo "gstack-browser-ensure: cannot resolve the install root" >&2; exit 1; }
 DOCS="https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#browser-ensure-failed"
diff --git a/bin/gstack-codex-probe b/bin/gstack-codex-probe
index 11f65f58b..0292e5674 100755
--- a/bin/gstack-codex-probe
+++ b/bin/gstack-codex-probe
@@ -157,7 +157,7 @@ _gstack_codex_select_model() {
   # working-directory context forwarded to the shared role resolver.
   # The TOML is read by the TypeScript resolver; bash never parses it.
   local _kind="${1:-exec}" _explicit="${2:-}" _role="${3:-}" _cwd="${4:-}" _line _rc _rest
-  local GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+  local GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
   case "$_kind" in exec|review) ;; *) echo "ERROR: _gstack_codex_select_model takes exec or review" >&2; return 2 ;; esac
   case "$_role" in ''|plan-review) ;; *) echo "ERROR: _gstack_codex_select_model role must be plan-review" >&2; return 2 ;; esac
   if ! command -v bun >/dev/null 2>&1; then
diff --git a/bin/gstack-codex-session-import b/bin/gstack-codex-session-import
index 88443d389..73543e799 100755
--- a/bin/gstack-codex-session-import
+++ b/bin/gstack-codex-session-import
@@ -20,7 +20,7 @@
 # Writes via bin/gstack-question-log so source tagging, dedup, and async
 # derive all apply uniformly.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 . "$(dirname "$0")/gstack-state-root.sh" 2>/dev/null || { echo "$0: cannot resolve the gstack state root: $(dirname "$0")/gstack-state-root.sh is missing. fix: reinstall with ./setup or /gstack-upgrade (docs/state-root.md)" >&2; exit 1; }
 gstack_state_root_select
diff --git a/bin/gstack-developer-profile b/bin/gstack-developer-profile
index de10bb92e..2380beb44 100755
--- a/bin/gstack-developer-profile
+++ b/bin/gstack-developer-profile
@@ -28,7 +28,7 @@
 # docs/designs/PLAN_TUNING_V0.md). Event file: ~/.gstack/projects/{SLUG}/
 # question-events.jsonl.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
@@ -176,7 +176,8 @@ EOF
 # and update aggregates (signals_accumulated, resources_shown, topics).
 # Fix for #1671: the writer side of the v1.0.0.0 migration. Reader and
 # writer now share the same file.
-# Silent skip on invalid input (matches gstack-timeline-log:22-26 pattern).
+# Invalid input is skipped silently; a Bun failure is reported and returned,
+# as in gstack-timeline-log (#3415).
 # -----------------------------------------------------------------------
 do_log_session() {
   local INPUT="${1:-}"
@@ -185,12 +186,18 @@ do_log_session() {
   fi
 
   # Validate: input must be parseable JSON with required fields (date, mode).
-  if ! printf '%s' "$INPUT" | bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e "
-    const j = JSON.parse(await Bun.stdin.text());
-    if (!j.date || !j.mode) process.exit(1);
-  " 2>/dev/null; then
-    return 0
-  fi
+  # Invalid input exits 3; any other non-zero exit means Bun itself failed.
+  local VALIDATE_RC=0
+  printf '%s' "$INPUT" | bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e "
+    let j;
+    try { j = JSON.parse(await Bun.stdin.text()); } catch { process.exit(3); }
+    if (!j || !j.date || !j.mode) process.exit(3);
+  " || VALIDATE_RC=$?
+  case "$VALIDATE_RC" in
+    0) ;;
+    3) return 0 ;;
+    *) echo "gstack-developer-profile: bun exited $VALIDATE_RC before the session was validated; nothing was logged" >&2; return "$VALIDATE_RC" ;;
+  esac
 
   ensure_profile
 
diff --git a/bin/gstack-distill-apply b/bin/gstack-distill-apply
index 013998183..769445d1c 100755
--- a/bin/gstack-distill-apply
+++ b/bin/gstack-distill-apply
@@ -23,7 +23,7 @@
 #   gstack-distill-apply --proposal <N> --gbrain-published true
 #   gstack-distill-apply --list                        # show pending proposals
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 . "$(dirname "$0")/gstack-state-root.sh" 2>/dev/null || { echo "$0: cannot resolve the gstack state root: $(dirname "$0")/gstack-state-root.sh is missing. fix: reinstall with ./setup or /gstack-upgrade (docs/state-root.md)" >&2; exit 1; }
 gstack_state_root_select
diff --git a/bin/gstack-distill-free-text b/bin/gstack-distill-free-text
index 85b87b743..2c4f50f67 100755
--- a/bin/gstack-distill-free-text
+++ b/bin/gstack-distill-free-text
@@ -20,7 +20,7 @@
 # auditability via --status when you want it.
 # Per D6: Anthropic SDK direct call, fail-loud on missing ANTHROPIC_API_KEY.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 # Heredoc delivery guard. bash 5.2+ writes a heredoc body <=64KiB through a
 # pipe in the forked child before exec, with no reader on the other end. On
diff --git a/bin/gstack-doctor b/bin/gstack-doctor
index e6ebdba85..9e18a5a38 100755
--- a/bin/gstack-doctor
+++ b/bin/gstack-doctor
@@ -40,7 +40,7 @@
 BASH_COMPAT=50
 
 set -u
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 usage() {
   sed -n '3,13p' "$0" | sed 's/^# \{0,1\}//'
diff --git a/bin/gstack-doctor-check.sh b/bin/gstack-doctor-check.sh
index 870607490..8cc9e2e34 100644
--- a/bin/gstack-doctor-check.sh
+++ b/bin/gstack-doctor-check.sh
@@ -19,7 +19,7 @@
 # remote's capy/support.env, `unknown` when its fetch failed, or ""), LIVE.
 # GSTACK_CAPY_WRAPPER_PATH (optional) names the wrapper a below-minimum fix runs.
 
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 . "$BIN/gstack-doctor-components.sh" || { echo "gstack-doctor: $BIN/gstack-doctor-components.sh is missing; reinstall with cd $SRC && ./setup" >&2; exit 1; }
 . "$BIN/gstack-install-registry.sh" 2>/dev/null || { echo "gstack-doctor: $BIN/gstack-install-registry.sh is missing; reinstall with cd $SRC && ./setup" >&2; exit 1; }
 . "$BIN/gstack-bun-version.sh" 2>/dev/null || { echo "gstack-doctor: $BIN/gstack-bun-version.sh is missing; reinstall with cd $SRC && ./setup" >&2; exit 1; }
diff --git a/bin/gstack-gbrain-install b/bin/gstack-gbrain-install
index c4b6fb269..4b67a12c6 100755
--- a/bin/gstack-gbrain-install
+++ b/bin/gstack-gbrain-install
@@ -36,7 +36,7 @@
 #   2 — prerequisite missing or invalid argument
 #   3 — post-install validation failed (PATH shadow, broken binary, etc.)
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 # --- defaults ---
 # No version pin by default — install the latest default-branch HEAD (#1744).
diff --git a/bin/gstack-gbrain-source-wireup b/bin/gstack-gbrain-source-wireup
index ab216c58d..97cbb1b4a 100755
--- a/bin/gstack-gbrain-source-wireup
+++ b/bin/gstack-gbrain-source-wireup
@@ -39,7 +39,7 @@
 # Depends on: bun (already a gstack dependency), gbrain, git.
 
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 CONFIG_BIN="$SCRIPT_DIR/gstack-config"
diff --git a/bin/gstack-hook-check b/bin/gstack-hook-check
index bffd2448b..a399d2ba6 100755
--- a/bin/gstack-hook-check
+++ b/bin/gstack-hook-check
@@ -37,7 +37,7 @@
 # (needed when a hook runs TypeScript) is not found. Set GSTACK_HOOK_CHECK_BUN to
 # the bun to run (setup passes its own).
 set -u
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 LIST_ONLY=0 SETUP_FILE=""
 while [ $# -gt 1 ]; do
diff --git a/bin/gstack-install-verdict.sh b/bin/gstack-install-verdict.sh
index 4fff5c06c..3e244e830 100644
--- a/bin/gstack-install-verdict.sh
+++ b/bin/gstack-install-verdict.sh
@@ -31,7 +31,7 @@
 # spawned sessions (ambient OpenClaw) only warn. GSTACK_ALLOW_UNSUPPORTED_RUNTIME=1
 # admits anyway and says so.
 
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 # gstack_source_root BIN_DIR — the source checkout behind an install: an
 # installed runtime root's recorded .source-path (setup _link_runtime_dists),
diff --git a/bin/gstack-ios-qa-daemon b/bin/gstack-ios-qa-daemon
index 366f2c6b7..0a2f713c7 100755
--- a/bin/gstack-ios-qa-daemon
+++ b/bin/gstack-ios-qa-daemon
@@ -21,7 +21,7 @@
 # session tokens are outstanding.
 
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 # The --help heredoc is over 512 bytes; keep it off a pipe (test/heredoc-pipe-deadlock.test.ts).
 BASH_COMPAT=50
 
diff --git a/bin/gstack-ios-qa-mint b/bin/gstack-ios-qa-mint
index 8192c88a6..44cbb8ebf 100755
--- a/bin/gstack-ios-qa-mint
+++ b/bin/gstack-ios-qa-mint
@@ -10,7 +10,7 @@
 # Allowlist file: ~/.gstack/ios-qa-allowlist.json (mode 0600).
 
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 GSTACK_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
diff --git a/bin/gstack-ios-qa-regen b/bin/gstack-ios-qa-regen
index ea4451cc5..33e6c95b9 100755
--- a/bin/gstack-ios-qa-regen
+++ b/bin/gstack-ios-qa-regen
@@ -3,7 +3,7 @@
 # package and the app-owned typed state accessors.
 
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 usage() {
   cat <<'EOF'
diff --git a/bin/gstack-learnings-log b/bin/gstack-learnings-log
index 117e5b68b..f5fa21713 100755
--- a/bin/gstack-learnings-log
+++ b/bin/gstack-learnings-log
@@ -6,7 +6,7 @@
 # Append-only storage. Duplicates (same key+type) are resolved at read time
 # by gstack-learnings-search ("latest winner" per key+type).
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 # Windows git-bash (#1950): pwd yields a POSIX path (/c/Users/...), which Bun
 # on Windows cannot resolve as an ES module specifier in the import below.
diff --git a/bin/gstack-learnings-search b/bin/gstack-learnings-search
index 8e45cd768..8e19da27c 100755
--- a/bin/gstack-learnings-search
+++ b/bin/gstack-learnings-search
@@ -8,7 +8,7 @@
 # exits 127 and a failed read exits non-zero with its stderr (#2790): "could
 # not read" must never look like "nothing recorded".
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
 . "$(dirname "$0")/gstack-state-root.sh" 2>/dev/null || { echo "$0: cannot resolve the gstack state root: $(dirname "$0")/gstack-state-root.sh is missing. fix: reinstall with ./setup or /gstack-upgrade (docs/state-root.md)" >&2; exit 1; }
diff --git a/bin/gstack-memorable b/bin/gstack-memorable
index 035723886..558767b14 100755
--- a/bin/gstack-memorable
+++ b/bin/gstack-memorable
@@ -32,7 +32,7 @@
 # Heredoc delivery guard (see bin/gstack-settings-hook for the rationale).
 BASH_COMPAT=50
 set -uo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
diff --git a/bin/gstack-question-log b/bin/gstack-question-log
index 1de42718d..6661ba8a8 100755
--- a/bin/gstack-question-log
+++ b/bin/gstack-question-log
@@ -31,7 +31,7 @@
 #
 # Append-only JSONL. Dedup is at read time in gstack-question-sensitivity --read-log.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 # Windows git-bash (#1950): pwd yields a POSIX path (/c/Users/...), which Bun
 # on Windows cannot resolve as an ES module specifier in bun -e imports.
diff --git a/bin/gstack-question-preference b/bin/gstack-question-preference
index 905e46677..5d0d1af40 100755
--- a/bin/gstack-question-preference
+++ b/bin/gstack-question-preference
@@ -35,7 +35,7 @@
 # --check already ignores those prefs; storing them made --stats lie.
 # always-ask on a one-way id is fine (it agrees with the safety override).
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
diff --git a/bin/gstack-relink b/bin/gstack-relink
index f4633fb38..f7443deba 100755
--- a/bin/gstack-relink
+++ b/bin/gstack-relink
@@ -9,7 +9,7 @@
 #   GSTACK_INSTALL_DIR — override gstack install directory
 #   GSTACK_SKILLS_DIR  — override target skills directory
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 GSTACK_CONFIG="${SCRIPT_DIR}/gstack-config"
diff --git a/bin/gstack-review-log b/bin/gstack-review-log
index c77b8882a..b60832497 100755
--- a/bin/gstack-review-log
+++ b/bin/gstack-review-log
@@ -17,7 +17,7 @@
 # a model read the code. Caller-supplied binding fields are always discarded.
 # Plan-tier rows retain their legacy binding behavior.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 export GIT_OPTIONAL_LOCKS=0
 export GIT_CONFIG_PARAMETERS="${GIT_CONFIG_PARAMETERS:+$GIT_CONFIG_PARAMETERS }'core.fsmonitor=false' 'core.untrackedCache=false'"
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
diff --git a/bin/gstack-review-read b/bin/gstack-review-read
index 2ebef6282..6be765749 100755
--- a/bin/gstack-review-read
+++ b/bin/gstack-review-read
@@ -9,7 +9,7 @@
 # rows using review_freshness; legacy or uncaptured diff evidence cannot fall
 # back to HEAD equality. Plan rows are unchanged.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 case "$(uname -s)" in
   MINGW*|MSYS*|CYGWIN*) command -v cygpath >/dev/null 2>&1 && SCRIPT_DIR="$(cygpath -m "$SCRIPT_DIR")" ;;
diff --git a/bin/gstack-settings-hook b/bin/gstack-settings-hook
index c5ca45a3b..239be557f 100755
--- a/bin/gstack-settings-hook
+++ b/bin/gstack-settings-hook
@@ -69,7 +69,7 @@
 BASH_COMPAT=50
 
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 ACTION="${1:-}"
 SETTINGS_FILE="${GSTACK_SETTINGS_FILE:-${CLAUDE_CONFIG_DIR:-$HOME/.claude}/settings.json}"
diff --git a/bin/gstack-specialist-stats b/bin/gstack-specialist-stats
index 144d1c317..0c5c66532 100755
--- a/bin/gstack-specialist-stats
+++ b/bin/gstack-specialist-stats
@@ -8,7 +8,7 @@
 # Fix-range passes (scope "fix-range", #3339) are skipped: a narrowed dispatch
 # says nothing about a specialist's hit rate on a whole branch.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
 . "$(dirname "$0")/gstack-state-root.sh" 2>/dev/null || { echo "$0: cannot resolve the gstack state root: $(dirname "$0")/gstack-state-root.sh is missing. fix: reinstall with ./setup or /gstack-upgrade (docs/state-root.md)" >&2; exit 1; }
diff --git a/bin/gstack-team-init b/bin/gstack-team-init
index b93cf25f6..d284ad04c 100755
--- a/bin/gstack-team-init
+++ b/bin/gstack-team-init
@@ -18,7 +18,7 @@
 BASH_COMPAT=50
 
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 MODE="${1:-}"
 
diff --git a/bin/gstack-telemetry-log b/bin/gstack-telemetry-log
index 9d518da7b..a662e350f 100755
--- a/bin/gstack-telemetry-log
+++ b/bin/gstack-telemetry-log
@@ -21,7 +21,7 @@
 #
 # NOTE: Uses set -uo pipefail (no -e) — telemetry must never exit non-zero
 set -uo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 GSTACK_DIR="${GSTACK_DIR:-$(cd "$(dirname "$0")/.." && pwd)}"
 SCRIPT_DIR="$GSTACK_DIR/bin"
diff --git a/bin/gstack-timeline-log b/bin/gstack-timeline-log
index 6bf54ceda..894c18082 100755
--- a/bin/gstack-timeline-log
+++ b/bin/gstack-timeline-log
@@ -10,7 +10,7 @@
 # Optional: branch, outcome, duration_s, session, ts.
 # Validation failure → skip silently (non-blocking).
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 eval "$("$SCRIPT_DIR/gstack-slug" 2>/dev/null)"
 . "$(dirname "$0")/gstack-state-root.sh" 2>/dev/null || { echo "$0: cannot resolve the gstack state root: $(dirname "$0")/gstack-state-root.sh is missing. fix: reinstall with ./setup or /gstack-upgrade (docs/state-root.md)" >&2; exit 1; }
diff --git a/bin/gstack-timeline-read b/bin/gstack-timeline-read
index 361af6f19..35db5fb46 100755
--- a/bin/gstack-timeline-read
+++ b/bin/gstack-timeline-read
@@ -10,7 +10,7 @@
 # --branch, or an unknown argument exits 2 before any read (#1723); --help
 # prints the usage line and exits 0.
 set -euo pipefail
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 USAGE='Usage: gstack-timeline-read [--since "7 days ago"|7d|YYYY-MM-DD] [--limit N] [--branch NAME]'
 SINCE_FORMS='accepted: "<N> minute|hour|day|week|month[s] ago", "<N>h|d|w", or an ISO date YYYY-MM-DD or YYYY-MM-DDTHH:MM:SSZ'
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
diff --git a/browse/test/learnings-injection.test.ts b/browse/test/learnings-injection.test.ts
index fe78e2618..2d202dca1 100644
--- a/browse/test/learnings-injection.test.ts
+++ b/browse/test/learnings-injection.test.ts
@@ -6,17 +6,22 @@ import { spawnSync } from 'child_process';
 const SCRIPT_PATH = path.join(import.meta.dir, '../../bin/gstack-learnings-search');
 const SCRIPT = fs.readFileSync(SCRIPT_PATH, 'utf-8');
 const BIN_DIR = path.join(import.meta.dir, '../../bin');
+const BUN_BLOCK_AT = SCRIPT.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e');
 
 describe('gstack-learnings-search injection safety', () => {
+  it('finds the bun -e block the checks below read', () => {
+    expect(BUN_BLOCK_AT).toBeGreaterThanOrEqual(0);
+  });
+
   it('must not interpolate variables into JS string literals', () => {
-    const jsBlock = SCRIPT.slice(SCRIPT.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e'));
+    const jsBlock = SCRIPT.slice(BUN_BLOCK_AT);
     expect(jsBlock).not.toMatch(/const \w+ = '\$\{/);
     expect(jsBlock).not.toMatch(/= \$\{[A-Z_]+\};/);
     expect(jsBlock).not.toMatch(/'\$\{CROSS_PROJECT\}'/);
   });
 
   it('must use process.env for parameters', () => {
-    const jsBlock = SCRIPT.slice(SCRIPT.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e'));
+    const jsBlock = SCRIPT.slice(BUN_BLOCK_AT);
     expect(jsBlock).toContain('process.env');
   });
 });
diff --git a/gstack-upgrade/migrations/v1.86.0.0.sh b/gstack-upgrade/migrations/v1.86.0.0.sh
index 133b3e9e4..ceb589c23 100755
--- a/gstack-upgrade/migrations/v1.86.0.0.sh
+++ b/gstack-upgrade/migrations/v1.86.0.0.sh
@@ -5,7 +5,7 @@
 # shared old renders are retired. This post-setup pass repairs missed upgrades.
 # Idempotent and non-fatal: foreign entries and failed replacements stay intact.
 set -u
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 INSTALL_DIR="${GSTACK_INSTALL_DIR:-$HOME/.claude/skills/gstack}"
 if [ -f "$INSTALL_DIR/bin/gstack-migrate-claude-code" ] && command -v bun >/dev/null 2>&1; then
   bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$INSTALL_DIR/bin/gstack-migrate-claude-code" --install-dir "$INSTALL_DIR" || true
diff --git a/hosts/claude/hooks/auq-error-fallback-hook b/hosts/claude/hooks/auq-error-fallback-hook
index 555e6cb55..7c351fd9b 100755
--- a/hosts/claude/hooks/auq-error-fallback-hook
+++ b/hosts/claude/hooks/auq-error-fallback-hook
@@ -3,6 +3,6 @@
 # wrapper makes the TypeScript hook executable via bun. Settings.json
 # references this file directly.
 set -e
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 HERE="$(cd "$(dirname "$0")" && pwd)"
 exec bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$HERE/auq-error-fallback-hook.ts"
diff --git a/hosts/claude/hooks/memorable-user-prompt-hook b/hosts/claude/hooks/memorable-user-prompt-hook
index 3e03ed734..9a7d07069 100755
--- a/hosts/claude/hooks/memorable-user-prompt-hook
+++ b/hosts/claude/hooks/memorable-user-prompt-hook
@@ -12,7 +12,7 @@
 # vendor process running with the prompt on its stdin. Forwarded, the signal
 # reaches the .ts, which kills the vendor's process group on its way out.
 # `<&0` keeps stdin: bash hands background jobs /dev/null otherwise.
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 HERE="$(cd "$(dirname "$0")" && pwd)" || exit 0
 bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$HERE/memorable-user-prompt-hook.ts" <&0 &
 _child=$!
diff --git a/hosts/claude/hooks/question-log-hook b/hosts/claude/hooks/question-log-hook
index d39baa681..c70484a90 100755
--- a/hosts/claude/hooks/question-log-hook
+++ b/hosts/claude/hooks/question-log-hook
@@ -3,6 +3,6 @@
 # wrapper makes the TypeScript hook executable via bun. Settings.json
 # references this file directly.
 set -e
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 HERE="$(cd "$(dirname "$0")" && pwd)"
 exec bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$HERE/question-log-hook.ts"
diff --git a/hosts/claude/hooks/question-preference-hook b/hosts/claude/hooks/question-preference-hook
index 7feb65c79..a910c3cf4 100755
--- a/hosts/claude/hooks/question-preference-hook
+++ b/hosts/claude/hooks/question-preference-hook
@@ -3,6 +3,6 @@
 # wrapper makes the TypeScript hook executable via bun. Settings.json
 # references this file directly.
 set -e
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 HERE="$(cd "$(dirname "$0")" && pwd)"
 exec bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$HERE/question-preference-hook.ts"
diff --git a/hosts/claude/hooks/timeline-stop-hook b/hosts/claude/hooks/timeline-stop-hook
index 71c75ae32..9cddf1fe5 100755
--- a/hosts/claude/hooks/timeline-stop-hook
+++ b/hosts/claude/hooks/timeline-stop-hook
@@ -5,7 +5,7 @@
 #
 # FAIL-OPEN (F5): a Stop-event telemetry repair must never block the session.
 # Every failure path — bun missing, script crash — still exits 0.
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 HERE="$(cd "$(dirname "$0")" && pwd)" || exit 0
 bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$HERE/timeline-stop-hook.ts" || true
 exit 0
diff --git a/lib/bun-isolation.ts b/lib/bun-isolation.ts
index c1e514a80..426b6dd3f 100644
--- a/lib/bun-isolation.ts
+++ b/lib/bun-isolation.ts
@@ -7,7 +7,8 @@
  * --config: shebangs and skill prose spell it
  * `bun --no-env-file --config=/dev/null`, shipped shell files spell
  * `--config="${GSTACK_BUN_CONFIG:-/dev/null}"` (NUL when OSTYPE is msys or
- * cygwin, #3415), TypeScript spawns use bunIsolationArgs().
+ * cygwin and cygpath is on PATH, #3415), TypeScript spawns use
+ * bunIsolationArgs().
  * test/bun-entry-isolation.test.ts holds every entry point to that form.
  *
  * POSIX passes /dev/null. Native Windows has no /dev/null and `--config=NUL`
diff --git a/plan-ceo-review/bin/mode-handoff-hook b/plan-ceo-review/bin/mode-handoff-hook
index a2656d6f1..58ad95c08 100755
--- a/plan-ceo-review/bin/mode-handoff-hook
+++ b/plan-ceo-review/bin/mode-handoff-hook
@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # Bash shim — Claude Code runs hook commands through /bin/sh; this runs the
 # TypeScript hook with bun.
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 HERE="$(cd "$(dirname "$0")" && pwd)"
 exec bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" "$HERE/mode-handoff-hook.ts"
diff --git a/scripts/build-cso.sh b/scripts/build-cso.sh
index c492bd920..303d03e6a 100644
--- a/scripts/build-cso.sh
+++ b/scripts/build-cso.sh
@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 set -eu
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 
 CSO_BUILD_ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 cd "$CSO_BUILD_ROOT"
diff --git a/scripts/lib/windows-curation.ts b/scripts/lib/windows-curation.ts
index 8e383e292..36dae63ac 100644
--- a/scripts/lib/windows-curation.ts
+++ b/scripts/lib/windows-curation.ts
@@ -314,7 +314,7 @@ export const KNOWN_WINDOWS_SAFE: Array<{ file: string; reason: string }> = [
   { file: 'test/ship-hook-windows-paths.test.ts', reason: 'runs bin/ helpers through explicit bash and Bun argv with forward-slash paths; never executes a shebang; the path-spelling simulation is skipIf win32' },
   { file: 'test/gstack-wtree-windows-paths.test.ts', reason: '#3101: runs bin/gstack-wtree through explicit bash in an os.tmpdir linked worktree with forward-slash paths; never executes a shebang; the drive-letter simulation is skipIf win32' },
   { file: 'test/setup-hook-syntax-gate.test.ts', reason: '#3088/#3100: runs bin/gstack-hook-check and the setup hook sections through explicit bash with forward-slash paths; never executes a shebang. Its Bun drive-path spelling is the Windows bug it guards' },
-  { file: 'test/bun-config-shell-bins.test.ts', reason: '#3415: runs the record bins through lib/script-invocation.ts (Git Bash on Windows) with MSYS path conversion off, the case it guards; its uname/bun stand-ins are skipIf win32' },
+  { file: 'test/bun-config-shell-bins.test.ts', reason: '#3415: runs the record bins through lib/script-invocation.ts (Git Bash on Windows) with MSYS path conversion off, the case it guards; its bun and cygpath stand-ins and simulated-OSTYPE cases are skipIf win32' },
   { file: 'test/path-spelling.test.ts', reason: 'sources bin/gstack-path-spelling.sh through explicit bash and checks lib/path-spelling.ts; on Windows it exercises the real Git Bash spellings' },
   { file: 'test/state-root-parity.test.ts', reason: 'runs the bash twin and lib/state-root.ts over an env table with PATH empty; no shebang execution, raw-string comparison is platform-neutral' },
   { file: 'test/generator-eexist.test.ts',
diff --git a/setup b/setup
index 43db8ec89..50225c258 100755
--- a/setup
+++ b/setup
@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # gstack setup — build browser binary + register skills with Claude Code / Codex
 set -e
-GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac
+GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac
 umask 077  # Restrict new files to owner-only (0o600 files, 0o700 dirs)
 
 # Heredoc delivery guard. bash 5.2+ writes a heredoc body <=64KiB through a
diff --git a/test/bun-config-shell-bins.test.ts b/test/bun-config-shell-bins.test.ts
index 01968aab6..a8d60f577 100644
--- a/test/bun-config-shell-bins.test.ts
+++ b/test/bun-config-shell-bins.test.ts
@@ -1,8 +1,8 @@
 /**
  * #3415: gstack's shell bins start Bun with
  * --config="${GSTACK_BUN_CONFIG:-/dev/null}", and each defines
- * GSTACK_BUN_CONFIG as NUL when OSTYPE is msys or cygwin (Git Bash, MSYS2,
- * Cygwin). Native bun.exe cannot
+ * GSTACK_BUN_CONFIG as NUL when OSTYPE is msys or cygwin and cygpath is on PATH
+ * (Git Bash, MSYS2, Cygwin). Native bun.exe cannot
  * open /dev/null (EINVAL), so a Windows bash with MSYS path conversion off
  * (MSYS_NO_PATHCONV=1, MSYS2_ARG_CONV_EXCL=*) made the literal spelling fail:
  * the record bins wrote nothing, and gstack-timeline-log still exited 0.
@@ -10,10 +10,12 @@
  * On every platform the free lanes run, windows-latest included, the record
  * bins run through lib/script-invocation.ts with conversion off in a hostile
  * repository: each writes its record, and the repository's bunfig.toml preload
- * and .env never load. On POSIX a simulated Windows bash (OSTYPE=msys, a bun
- * that rejects /dev/null the way bun.exe does) reproduces the bug; its
- * controls, without OSTYPE=msys, show each bin fails loudly with nothing
- * written instead of exiting 0.
+ * and .env never load. On POSIX a simulated Windows bash (OSTYPE=msys, a
+ * cygpath, a bun that rejects /dev/null the way bun.exe does) reproduces the
+ * bug; its controls, without OSTYPE=msys, show each bin fails loudly with
+ * nothing written instead of exiting 0. An OSTYPE=msys inherited by a POSIX
+ * bash with no cygpath keeps /dev/null, so a file named NUL in the repository
+ * is never read as Bun's config.
  */
 import { afterAll, beforeAll, describe, expect, test } from 'bun:test';
 import { spawnSync } from 'node:child_process';
@@ -47,8 +49,8 @@ function env(pathPrefix?: string, extra: Record<string, string> = {}): NodeJS.Pr
   return { ...out, HOME: path.join(tmp, 'home'), GSTACK_STATE_ROOT: stateRoot, MSYS_NO_PATHCONV: '1', MSYS2_ARG_CONV_EXCL: '*', ...extra };
 }
 
-function run(bin: string, input: string, pathPrefix?: string, extra: Record<string, string> = {}) {
-  return runScriptSync(path.join(ROOT, 'bin', bin), [input], { cwd: repo, env: env(pathPrefix, extra), encoding: 'utf8', timeout: 60_000 });
+function run(bin: string, input: string, pathPrefix?: string, extra: Record<string, string> = {}, rest: string[] = []) {
+  return runScriptSync(path.join(ROOT, 'bin', bin), [input, ...rest], { cwd: repo, env: env(pathPrefix, extra), encoding: 'utf8', timeout: 60_000 });
 }
 
 function records(log: string): string[] {
@@ -99,6 +101,7 @@ beforeAll(() => {
     `exec ${JSON.stringify(process.execPath)} "\${a[@]}"`,
     '',
   ].join('\n'));
+  writeExec(path.join(fakeBun, 'cygpath'), '#!/usr/bin/env bash\nprintf \'%s\\n\' "${@: -1}"\n');
 });
 
 afterAll(() => { if (tmp) fs.rmSync(tmp, { recursive: true, force: true }); });
@@ -135,6 +138,20 @@ describe('#3415: record bins start Bun on a Windows bash with path conversion of
 });
 
 describe.skipIf(IS_WINDOWS)('#3415: simulated Windows bash (POSIX hosts)', () => {
+  test('gstack-developer-profile --log-session reports a Bun failure instead of skipping it, and still skips invalid input silently', () => {
+    resetState();
+    const session = JSON.stringify({ date: '2026-10-25', mode: 'startup' });
+    const failed = run('gstack-developer-profile', '--log-session', fakeBun, {}, [session]);
+    expect(failed.stderr).toContain('EINVAL');
+    expect(failed.stderr).toContain('nothing was logged');
+    expect(failed.status).not.toBe(0);
+    for (const bad of ['not json', '{"date":"2026-10-25"}', 'null']) {
+      const r = run('gstack-developer-profile', '--log-session', undefined, {}, [bad]);
+      expect(r.status).toBe(0);
+      expect(r.stderr).toBe('');
+    }
+  });
+
   for (const { bin, input, log } of BINS) {
     test(`${bin} passes NUL and writes its record`, () => {
       resetState();
@@ -145,6 +162,21 @@ describe.skipIf(IS_WINDOWS)('#3415: simulated Windows bash (POSIX hosts)', () =>
       expect(fs.existsSync(preloadMarker)).toBe(false);
     });
 
+    test(`an inherited OSTYPE=msys without cygpath keeps /dev/null: ${bin} never reads a repository file named NUL`, () => {
+      resetState();
+      const nul = path.join(repo, 'NUL');
+      fs.writeFileSync(nul, 'preload = ["./preload.ts"]\n');
+      try {
+        const r = run(bin, input, undefined, { OSTYPE: 'msys' });
+        expect(r.status).toBe(0);
+        expect(records(log).length).toBe(1);
+        expect(fs.existsSync(preloadMarker)).toBe(false);
+      } finally {
+        fs.rmSync(nul, { force: true });
+        fs.rmSync(preloadMarker, { force: true });
+      }
+    });
+
     test(`control: when Bun rejects the config, ${bin} fails loudly and writes nothing`, () => {
       resetState();
       const r = run(bin, input, fakeBun);
diff --git a/test/bun-entry-isolation.test.ts b/test/bun-entry-isolation.test.ts
index 15ad346fc..64b35ce4a 100644
--- a/test/bun-entry-isolation.test.ts
+++ b/test/bun-entry-isolation.test.ts
@@ -30,7 +30,7 @@ const ROOT = path.resolve(import.meta.dir, '..');
 const FLAGS = '--no-env-file --config=/dev/null';
 const ISOLATED_SHEBANG = `#!/usr/bin/env -S bun ${FLAGS}`;
 const SHELL_FLAGS = '--no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}"';
-const CONFIG_DEF = 'GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac';
+const CONFIG_DEF = 'GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac';
 
 const TRACKED = spawnSync('git', ['ls-files', '-z'], { cwd: ROOT, encoding: 'utf8', timeout: 30_000 }).stdout.split('\0').filter(Boolean)
   .filter(f => !/(^|\/)test\//.test(f) && fs.existsSync(path.join(ROOT, f)) && fs.statSync(path.join(ROOT, f)).isFile());
diff --git a/test/bun-isolation-launch.test.ts b/test/bun-isolation-launch.test.ts
index 8ed07db1f..d56a943bf 100644
--- a/test/bun-isolation-launch.test.ts
+++ b/test/bun-isolation-launch.test.ts
@@ -18,7 +18,7 @@ import { bunIsolationArgs } from '../lib/bun-isolation';
 import { bashCommandInvocation } from '../lib/script-invocation';
 
 const PRINT = 'console.log("env=" + (process.env.GSTACK_LAUNCH_PROBE ?? "unset"))';
-const CONFIG_DEF = 'GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) GSTACK_BUN_CONFIG=NUL ;; esac';
+const CONFIG_DEF = 'GSTACK_BUN_CONFIG=/dev/null; case "${OSTYPE:-}" in msys*|cygwin*) if command -v cygpath >/dev/null 2>&1; then GSTACK_BUN_CONFIG=NUL; fi ;; esac';
 const NO_PATHCONV = { MSYS_NO_PATHCONV: '1', MSYS2_ARG_CONV_EXCL: '*' };
 let project = '';
 let preloadMarker = '';
diff --git a/test/learnings-injection.test.ts b/test/learnings-injection.test.ts
index d542c6d01..89515fe79 100644
--- a/test/learnings-injection.test.ts
+++ b/test/learnings-injection.test.ts
@@ -6,10 +6,15 @@ const SCRIPT = path.join(import.meta.dir, "..", "bin", "gstack-learnings-search"
 
 describe("gstack-learnings-search injection prevention", () => {
   const script = readFileSync(SCRIPT, "utf-8");
+  const bunBlockAt = script.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e "');
+
+  test("the bun -e block the checks below read is present", () => {
+    expect(bunBlockAt).toBeGreaterThanOrEqual(0);
+  });
 
   test("no shell interpolation inside bun -e string", () => {
     // Extract the bun -e block (everything between `bun -e "` and the closing `"`)
-    const bunBlock = script.slice(script.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e "'));
+    const bunBlock = script.slice(bunBlockAt);
 
     // Should NOT contain ${VAR} patterns (shell interpolation)
     // These are RCE vectors: a malicious learnings entry with '; rm -rf / ;' in the
@@ -27,7 +32,7 @@ describe("gstack-learnings-search injection prevention", () => {
   });
 
   test("uses process.env for all user-controlled values", () => {
-    const bunBlock = script.slice(script.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e "'));
+    const bunBlock = script.slice(bunBlockAt);
 
     // Must use process.env for TYPE, QUERY, LIMIT.
     // SLUG and CROSS are no longer threaded as env vars inside the bun
@@ -52,7 +57,7 @@ describe("gstack-learnings-search injection prevention", () => {
   });
 
   test("current vs cross-project rows distinguished by inline tags, not SLUG env (#1619)", () => {
-    const bunBlock = script.slice(script.indexOf('bun --no-env-file --config="${GSTACK_BUN_CONFIG:-/dev/null}" -e "'));
+    const bunBlock = script.slice(bunBlockAt);
     // The bun block must inspect the per-line tag to mark cross-project rows.
     // The current shape emits `current\t<json>` or `cross\t<json>` from the
     // upstream pipe (via emit_tagged_file). Inside the bun block, the script